Benton M.J. Introduction to paleobiology and the fossil record

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348 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

whereas Gastrioceras was a depressed, tuber-

culate form.

The order Ceratitida includes the suborders

Prolecantida and Ceratitida. The prolecanti-

dines (Early Carboniferous to Late Permian)

had large, smooth shells with wide umbilici,

and sutures grading from goniatitic to cera-

titic. Prolecanites, for example, was evolute

with a wide umbilicus. The ceratitides include

most of the Triassic ammonoids with ceratitic

suture patterns and commonly elaborate

ornamented shells. Nevertheless, some taxa

developed ammonitic-grade sutures and a

number of lineages evolved heteromorphs

(Box 13.7).

The ammonites proper (Fig. 13.18) com-

prise four orders, the Phylloceratida, the

Lytoceratida, the Ammonitida and the Ancy-

loceratida. The ammonitides appeared ﬁ rst in

the Early Triassic with ammonitic sutures,

commonly ornamented shells and ventral sip-

huncles. The ﬁ rst members of the order Phyl-

loceratida, such as Leiophyllites, appear in

Lower Triassic faunas and, according to some,

this stem group probably gave rise to the

entire ammonite fauna of the Jurassic and

Cretaceous (Fig. 13.19). The morphologically

conservative Phylloceras survived from the

Early Jurassic to near the end of the Creta-

ceous with virtually no change, after having

generated many of the major post-Triassic lin-

eages. The phylloceratides were smooth, invo-

lute (with the last whorl covering all the

previous ones), compressed forms; the suture

had a marked leaf-like or phylloid saddle and

a crook-shaped or lituid internal lobe.

Although the group had a near-cosmopolitan

distribution, its members were most common

in the Tethyan province, but were character-

istic of open-water environments.

The lytoceratides originated near the base

of the Jurassic, with evolute (all previous

whorls visible), loosely coiled shells, as

seen in Lytoceras itself, which had a near-

cosmopolitan distribution particularly during

high stands of sea level. Like the phyllocera-

tides, the order remained conservative;

however, it too generated many other groups

of Jurassic and Cretaceous ammonites.

The ammonitides included the true ammo-

nites and ranged from the Lower Jurassic to

the Upper Cretaceous, whereas the ancyloc-

eratides included most of the bizarre hetero-

morph ammonites, ranging from the Upper

Jurassic to the Upper Cretaceous.

ammonitic

ceratitic

goniatitic

agoniatitic

orthoceratitic

Figure 13.17 Evolution of suture patterns: the

ﬁ ve main types; arrows point towards the frontal

aperture.

Figure 13.18 (opposite) Ammonite taxa: (a)

Ludwigia murchisonae (macroconch) from the

Jurassic of Skye, (b) cluster of Ludwigia

murchisonae (microconchs) from the Jurassic of

Skye, (c) Quenstedtoceras henrici from the

Jurassic of Wiltshire, (d) Quenstedtoceras henrici

(showing a characteristic suture pattern) from

the Jurassic of Wiltshire, and (e) Peltomorphites

subtense from the Jurassic of Wiltshire, (f)

Placenticeras (Cretaceous), (g) Lytoceras

(Jurassic), (h) Hildoceras (Jurassic) and (i)

Cadoceras (Cretaceous). Magniﬁ cation ×1 (a–e),

×0.5 (f–i). (a–e, courtesy of Neville

Hollingworth.)

(a) (b)

(f) (g)

(h) (i)

350 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

Time (Ma)

CenozoicMesozoicPaleozoic

Era Period

Quaternary

Tertiary

Cretaceous

Jurassic

Triassic

Permian

Carboniferous

Devonian

Silurian

Ordovician

Cambrian

1.5

130

190

220

280

340

400

440

500

570

Nautiloidea

Endoceratoidea

Actinoceratoidea

Teuthoidea

Sepioidea

Octopoda

Belemnitida

Bactritida

Plectronoceras

Clymeniida

Anarcestida

Aulacoceratida

Ancyloceratida

Lytoceratida

Phylloceratida

Ceratitida

Ammonitida

Prolecanitida

Goniatitida

Ammonoidea

Figure 13.19 Stratigraphic ranges of the main ammonite taxa together with the other main cephalopod

groups. (Based on Ward, P. 1987. Natural History of Nautilus. Allen & Unwin, Boston.)

Ammonoid ecology and evolution

The pioneer work by Arthur Trueman (Univer-

sity of Glasgow) on the buoyancy and orienta-

tion of the ammonite shell established the

probable life attitudes for even the most bizarre

heteromorph forms (Fig. 13.21a). Theoreti-

cally, at least, virtually all ammonoids could

favorably adjust their attitude and buoyancy

in the water column. Most ammonoids were

probably part of the mobile benthos, although

after death their gas-ﬁ lled shells could be

widely distributed by oceanic currents. Many

groups of ammonoids were endemic, and the

shovel-like jaws of some groups were most

efﬁ cient at the sediment–water interface.

Richard Batt’s studies (1993) on Cretaceous

ammonite morphotypes from the United States

have established a series of shell types related

to life modes and environments (Fig. 13.21b).

For example, evolute heavily ornamented

forms were probably nektobenthonic, as were

spiny cadicones and spherocones, nodose

spherocones and platycones, together with

broad cadicones. Evolute planulates and ser-

penticones, together with small planulates,

were probably pelagic in the upper parts of the

water column. However, most oxycones were

restricted to shallow-shelf depths. Some het-

eromorphs were nektobenthonic, whereas a

few ﬂ oated in the surface waters.

In many ammonite faunas the consistent

co-occurrence of large and small similarly

ornamented mature shells at speciﬁ c horizons

suggests that the macroconch and microconch

may be related sexual dimorphs (the male and

female of the species). The macroconch was

probably the female, though this may not

always have been the case. The ammonoids

probably originated from the bactritid ortho-

cone nautiloids, with protoconchs and large

body chambers, during the earliest Devonian.

The anarcestide goniatites, with simple sutural

patterns, were relatively scarce during the

Mid Devonian. However, by the Famennian,

other groups such as the clymeniids, with a

dorsally situated siphuncle, were common.

The goniatitides expanded during the

SPIRALIANS 2: MOLLUSKS 351

Box 13.7 Ammonite heteromorphs

One of the more spectacular aspects of ammonite evolution was the appearance of bizarre hetero-

morphic (“different shape”) shells in many lineages at a number of different times (Fig. 13.20).

Heteromorphs ﬁ rst appeared during the Devonian, but were particularly signiﬁ cant in Late Triassic

and Late Cretaceous faunas. Some such as Choristoceras, Leptoceras and Spiroceras appeared merely

to uncoil; Hamites, Macroscaphites and Scaphites partly uncoiled and developed U-bends; whereas

Noestlingoceras, Notoceras and Turrilites mimicked gastropods and Nipponites adopted shapes

based on a series of connected U-bends. Initially, the heteromorph was considered as a decadent

degenerate animal anticipating the extinction of a lineage. Nevertheless, some heteromorphs appar-

ently gave rise to more normally coiled descendants and their association with extinction events only

is far from true. Additionally, functional modeling suggests many were perfectly adapted to both

nektobenthonic and pelagic life modes. Moreover Stephane Reboulet and her colleagues (2005) have

shown that among the ammonites in the Albian rocks of the Vocontian Basin, southern France, het-

eromorphs probably were better adapted to compete in meso- and oligostrophic conditions than

many other groups.

Turrilites (Cretaceous)

Nipponites (Cretaceous)

Hamulina (Cretaceous)

Hyphantoceras

(Cretaceous)

Ostlingoceras

(Cretaceous)

Macroscaphites

(Cretaceous)

Spiroceras

(Jurassic)

Choristoceras

(Triassic)

Figure 13.20 Some heteromorph ammonites.

Carboniferous, together with the prolecan-

tides, where all the subsequent ammonoids

probably originated. During the Triassic, the

ceratitides diversiﬁ ed, peaking in the Late

Triassic; but by the Jurassic the smooth in-

volute phylloceratides, the lytoceratides and

the ammonitides were all well established.

Complex septa and sutures may have increased

the strengths of the ammonoid phragmocone,

protecting the shell against possible implosion

at deeper levels in the water column. More

intricate septa also provided a larger surface

area for the attachment of the soft parts of

the living animal, perhaps aiding more vigor-

ous movement of the animal and its shell.

Coleoidea

The subclass Coleoidea contains cuttleﬁ sh,

squids and octopuses, the latter including the

paper nautilus, Argonauta. Coleoids show the

dibranchiate condition, with a single pair of

gills within the mantle cavity. Although argo-

nauts can be traced back to the Mid Tertiary,

the living coleoid orders generally have a poor

fossil record, but preservation of arms, ink

352 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

sacs and body outline is well known from

several localities in the Jurassic. In contrast,

the skeletons of extinct belemnites are locally

abundant in Jurassic and Cretaceous rocks.

Belemnites had an internal skeleton, contrast-

ing with the exoskeletons of the shelled ceph-

alopods such as the nautiloids and ammonoids.

The belemnite skeleton is relatively simple,

consisting of three main parts: the bullet-

shaped guard is solid and composed of radi-

(a)

100 m

Ludwigia

Caloceras

Promicroceras

Normannites

Crioceras sp. 1

Crioceras sp. 3

Macroscaphites

Turrilites

Crioceras sp. 2

Dactylioceras

Sigaloceras

Scaphites

Oecoptychius

Lytocrioceras

oxygenated

anoxic

100 m 100 m

(b)

Figure 13.21 Life attitudes and buoyancy of the ammonites. (a) Supposed life orientations of a

selection of ammonite genera, with the center of gravity marked ×; the center of buoyancy is marked

with a dot and the extent of the body chamber is indicated with subparallel lines. (b) Relationship of

some ammonite morphotypes to water depth and the development of anoxia. (a, from Trueman, A.E.

1940. Q. J. Geol. Soc. Lond. 96; b, from Batt 1993.)

SPIRALIANS 2: MOLLUSKS 353

ally arranged needles of calcite with, at its

anterior end, a conical depression or alveolus

that houses the apical portion of the conical

phragmocone, consisting of concave septa

and a ventral siphuncle, and the spatulate

pro-ostracum (corresponding to the dorsal

wall of the body chamber of ectocochliate

forms) that extends anteriorly (Fig. 13.22).

This assemblage, situated on the dorsal side

of the animal, is analogous to the chambered

shells of nautiloids and ammonoids. Soft parts

of belemnites, including the contents of ink

sacs and tentacle hooks, are also occasionally

preserved.

ventral

(anterior)

head

anterior (dorsal)

dorsal

(posterior)

visceral hump

mantle cavity

opening of

mantle cavity

posterior (ventral)

(a)

arms

beak

radula

internal

remnant

of shell

digestive

diverticulum

cecum

gonad

heart

kidney

anus

mantle

cavity

funnel

brain

mouth

tentacle

(b)

(c)

(d)

Hibolites

Middle Jurassic – Late Cretaceous

Duvalia

Middle Jurassic – Late Cretaceous

Hastites

Late – Middle Jurassic

Actinocamax

Upper Cretaceous

Belemnitella

Upper Cretaceous

pro-ostracum phragmocone guard

Figure 13.22 Coleoid morphology: (a) reconstruction of a living belemnite, (b) soft-part morphology of

the belemnites, (c) internal skeleton of the belemnites, and (d) some belemnite genera. (From Peel et al.

1985.)

354 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

By analogy with modern squids, the belem-

nites were probably rapidly-moving predators

living in shoals with their body level regulated

by the guard. The animal thus probably main-

tained a horizontal attitude within the water

column, preferring the open ocean. Data from

the stomach contents of ichthyosaurs conﬁ rm

that these mollusks formed part of their

diet.

Some of the oldest records of belemnites,

for example Jeletzkya from the mid-

Carboniferous of Illinois, are tentative. The

ﬁ rst unequivocal belemnites are from the

Middle Triassic rocks of Sichuan Province,

China where several species of Sinobelemnites

occur. Belemnites became extinct at the end

of the Cretaceous; later records are reworked

or based on misinterpretations.

Some of the ﬁ rst supposed belemnites, like

the Carboniferous Paleoconus, were relatively

short stubby forms. In the Early Jurassic,

Megateuthis was a long, slender form, whereas

Dactyloteuthis was laterally ﬂ attened; the

later Jurassic Hibolites is spear-shaped. The

Cretaceous Belemnitella has a large bullet-

shaped guard, whereas that of Duvalia has a

ﬂ attened spatulate shape (Fig. 13.22d).

However, despite differences in the detailed

morphology of the endoskeltons across

genera, many authorities consider that most

of the Mesozoic belemnites probably looked

very similar, but there are still enough features

to measure on their skeletons and discrimi-

nate taxa (Box 13.8).

The compact calcareous guards of the bel-

emnites have proved ideal for the analysis of

oxygen isotope ratios (O

: O

) relating to

paleotemperature conditions in the Jurassic

and Cretaceous seas. These data have indi-

cated warm peaks during the Albian and the

Coniacian-Santonian (mid-Cretaceous) with a

gradual cooling from the Campanian (Late

Cretaceous) onwards. And as with many other

Mesozoic groups, belemnite distributions

show separate low-latitude Tethyan and high-

latitude Boreal assemblages.

Spectacular mass accumulations of belem-

nite rostra are relatively common in Mesozoic

sediments and, although some authors have

used these assemblages in paleocurrent studies,

few have addressed their mode of accumula-

tion. Dense accumulations of bullet-shaped

belemnite rostra have promoted the term

“belemnite battleﬁ elds” for such distinctive

shell beds (Fig. 13.23). These accumulations

conform to ﬁ ve genetic types (Doyle & Mac-

Donald 1993): (1) post-spawning mortalities

(Fig. 13.23a); (2) catastrophic mass mortali-

ties; (3) predation concentrates, either in situ

or regurgitated (Fig. 13.23b); (4) condensa-

tion deposits perhaps aided by winnowing

and sediment by-pass; and (5) resedimented

deposits derived from usually condensed

accumulations. Many of these so-called bel-

emnite battleﬁ elds are then partly natural

occurrences, reﬂ ecting the biology of the

animals (numbers 1–3), but it is important to

distinguish these from sedimentary accumula-

tions (numbers 4 and 5) that say nothing

about belemnite behavior.

CLASS SCAPHOPODA

Scaphopods are generally rare as fossils. The

Scaphopoda, or elephant-tusk shells, have a

single, slightly curved high conical shell, open

at both ends (Fig. 13.25a). They lack gills and

eyes, but have a mouth equipped with a radula

and surrounded by tentacles; they also possess

a foot, similar to that of the bivalves, adapted

for burrowing. Scaphopods are mainly car-

nivorous, feeding on small organisms such as

foraminiferans and spending much of their

life in quasi-infaunal positions within soft

sediment in deeper-water environments. The

ﬁ rst scaphopods appeared during the Devo-

nian and apparently had similar lifestyles to

living forms such as Dentalium.

CLASS ROSTROCONCHA

Relatively recently a small class of mollusks,

superﬁ cially resembling bivalves but lacking

a functional hinge, has been documented

from the Paleozoic. Over 35 genera have been

described; most were originally described as

bivalved arthropods. The rostroconchs prob-

ably had a foot that emerged through the

anterior gape between the shells. However,

the two shells are in fact fused along the mid-

dorsal line, and posteriorly the shells are

extended as a platform or rostrum (Fig.

13.25b). Ontogeny occurs from an initial dis-

soconch with the bilobed form developing

from the disproportionate growth of shell

from the lateral lobes of the mantle. The

group appeared ﬁ rst during the Early Cam-

brian when, for example, Heraultipegma and

SPIRALIANS 2: MOLLUSKS 355

Watsonella dominated rostroconch faunas of

the Tommotian. The rostroconchs diversiﬁ ed

during the Ordovician to reach an acme in the

Katian when all seven families were repre-

sented. They probably occupied similar eco-

logical niches to those of the bivalves.

However, there followed a decline in abun-

dance and diversity until ﬁ nal extinction at

the end of the Permian when only conocar-

diodes such as Arceodomus were still extant.

The rostroconchs occupy a pivotal position

in molluskan evolution (Runnegar & Pojeta

1974). The group developed from within the

monoplacophoran plexus with a loss of seg-

mentation; the rostroconchs themselves gen-

erated both the bivalves and the scaphopods

whereas the gastropods and cephalopods were

probably derived independently from a sepa-

rate monoplacophoran ancestor. In some

respects, the rostroconchs may represent a

missing link between the univalved and

bivalved molluskan lineages, while their

unlikely morphology may have contributed

towards their late discovery.

EVOLUTIONARY TRENDS WITHIN

THE MOLLUSCA

A spectacular variety of mollusk morphotypes

and life modes evolved during the Phanero-

zoic, from the simple body plan of the arche-

mollusk. Despite the diversity of early mollusks

in the Cambrian, the phylum was not notably

conspicuous in the tiered suspension-feeding

post-spawning mortality model: Antarctic example

spawning ground

post-spawning mortality

turbidity-current triggered

predation

foundered vertebrate with intact gastric mass

predation concentration model

regurgitates

sparse belemnites

belemnite-rich turbidite

accumulation and concentration

by shallow-marine processes

(a)

(b)

migration to spawning grounds

basin

limited mortality

shelf

Figure 13.23 Belemnite battleﬁ elds and their possible origin: (a) post-spawning mortality model and

(b) predation concentration model. (From Doyle & MacDonald 1993.)

356 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

Box 13.8 Gradualistic evolution of belemnites

There are relatively few long fossil lineages that can be used to demonstrate either phyletic gradual-

ism or punctuated equilibria. Most of the best case studies (see p. 124) are based on mobile or sessile

benthic organisms. The Cretaceous (Campanian) belemnite faunas, particularly the genus Belem-

nitella, of North Germany are abundant, well preserved and known in great detail and provide an

unequalled opportunity to test these models using a pelagic group of organisms (Christensen 2000).

There is not much variation in lithology throughout the succession in Lower Saxony – they are

mainly rather boring, monotonous marly limestones. There are thus limited opportunities for facies

shifts to inﬂ uence the morphological record of Belemnitella by the migration of more exotic mor-

photypes in and out of the basin. Although samples of Belemnitella through the section are superﬁ -

cially similar, several measurements show gradualistic trends when treated quantitatively (Fig. 13.24).

Not all changes are unidirectional, some exhibit reversals. It seems most likely that the Campanian

belemnites of northern Germany conformed to continuous, gradual phyletic evolution in narrowly

ﬂ uctuating, slowly changing environments.

LAP in mm BI SD in mm FA in degrees AA in degrees

(30)

(43)

(46)

(30)

(28)

(43)

(46)

(36)

(38)

(37)

(28)

(27)

(38)

(89)

(88)

(78)

(67)

(77)

(85)

(63)

(69)

(84)

(80)

(87)

(17)

(6)

(34)

(30)

(33)

(15)

(12)

(7)

(4)

(108)

(38)

(22)

(12)

(38)

(22)

(12)

20 30 40 50 60 2010 30 40 50 18 20 22 242.5 3.0 3.5 4.0 4.5 4 6 8 10 12 14

“Vulgaris”/

Stolleyi Zone

“Vulgaris”/

Basiplana Zone

Stobaei/

Basiplana Zone

Conica/

Mucronata Zone

Gracilis/

Mucronata Zone

Conica/

Papillosa Zone

Senonensis Zone

Pilula/

Senonensis Zone

Belemnitella mucronata

B.mis-

burgensis

Upper Lower Campanian

Lower Upper Campanian

mean value,

± 1 standard deviation

and observed range

(38) = sample size

20 m

Figure 13.24 Gradualistic evolution of Cretaceous belemnites from North Germany. Summary of

changes of the length from the apex to the protoconch (LAP), Birkelund index (BI), Schatzky

distance (SD), ﬁ ssure angle (FA) and alveolar angle (AA) of nine samples of Belemnitella.

Successive mean values are different at the 5% level (one arrow), 1% level (two arrows) and

0.1% level (three arrows). (Courtesy of the late Walter Kegel Christensen.)

SPIRALIANS 2: MOLLUSKS 357

benthos of the Paleozoic, although many more

localized, often nearshore, assemblages were

dominated by mollusks.

During the Paleozoic, bivalves were

common in nearshore environments, often

associated with lingulide brachiopods,

although the class also inhabited a range of

deeper-water clastic environments; and by the

Late Paleozoic bivalves had invaded a variety

of carbonate environments. However, at the

end of the Paleozoic, the appearance of more

typical bivalves in shallow-water belts may

have displaced the Paleozoic associations

seaward. During the Late Mesozoic and Ceno-

zoic the signiﬁ cant radiation of infaunal

taxa may have been a response to increased

predation.

The majority of Paleozoic gastropods were

Eogastropoda that commonly dominated

shallow-water marine environments and some

carbonate reef settings. The Mesozoic, however,

was dominated by the Mesogastropoda, which

grazed on algal-coated hard substrates. The

Cenozoic, marks the acme of the group with

the radiation of the siphonal carnivorous

neogastropods, and with a further diversiﬁ ca-

tion of mesogastropods (Fig. 13.26).

The cephalopods evolved through the

development of a chambered shell with a sip-

huncle, which gave them considerable control

over attitude and buoyancy; this system was

reﬁ ned in the nautiloid groups. The evolution

of complex folded sutures in the ammonoids,

the exploitation of a pelagic larval stage and

a marginal position for the siphuncle appar-

ently set the agenda for the further radiation

of the group during the Mesozoic.

Throughout the Phanerozoic, the ﬂ eshy

mollusks provided a source of nutrition for

many groups of predators. The evolution of

the phylum was probably in part inﬂ uenced

by the development of predator–prey rela-

tionships and minimization of predator

success. Thick armored shells were developed

in some groups while the evolution of deep-

infaunal life modes was also part of a defen-

sive strategy. Predation and the development

of avoidance strategies, together with the

so-called arms race, had an important inﬂ u-

ence on molluskan evolution. Predators

foot

head

(a)

gut

mantle

mantle cavity

water–sediment interface

kidney

anus

gonad

shell aperture

posterior

gape

rostrum

(b)

Figure 13.25 (a) Scaphopod morphology and

(b) rostroconch morphology.

Eras

Quaternary

Pliocene

Miocene

Oligocene

Eocene

Paleocene

Paleogene

Neogene

Tertiary

Cenozoic

Mesozoic

Cretaceous

Jurassic

Triassic

Lower Upper

Paleozoic

Permian

Carboniferous

Devonian

Silurian

Ordovician

Cambrian

Monoplacophora

Diplacophora

Helcionellida

Bivalvia

Rostroconchia

Tergomyonia

Cephalopoda

Polyplacophora

Gastropoda

Scaphopoda

Aplacophora

Periods

Figure 13.26 Stratigraphic range of the main

mollusk groups.